arXiv subjects

Chris Welch

Publications and source records attributed to Chris Welch.

8 recordsLinked to original sources

arXiv2023

Dipole-dipole correlations in the nematic phases of symmetric cyanobiphenyl dimers and their binary mixtures with 5CB

We report on the temperature dependence of birefringence and of the static dielectric permittivity tensor in a series of binary mixtures between the symmetric, bent-shaped, 1",9"-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB) dimer and the monomeric nematogen 5CB. In the studied composition range the mixtures exhibit two nematic phases with distinct birefringence and dielectric features. Birefringence measurements are used to estimate the temperature dependence of the tilt between the axis defining the nanoscale helical modulation of the low temperature nematic phase with the (local) direction of the maximal alignment of the cyanobiphenyl units. Planar as well as magnetically and/or electrically aligned samples are used to measure the perpendicular and parallel components of the dielectric permittivity in both nematic phases. A self-consistent molecular field theory that takes into account flexibility and symmetry of the constituent mesogens is introduced for the calculation of order parameters and intra-molecular orientational dipolar correlations of the flexible dimers as a function of temperature/concentration. Utilising the tilt angle, as calculated from the birefringence measurements, and the predictions of the molecular theory, dielectric permittivity is modelled in the framework of the anisotropic version of the Kirkwood-Frohlich theory. Using the inter-molecular Kirkwood correlation factors as adjustable parameters, excellent agreement between theory and permittivity measurements across the whole temperature range and composition of the mixtures is obtained. The importance of the orientational, intra- and inter-molecular, dipolar correlations, their relative impact on the static dielectric properties, as well as their connection with the local structure of the nematic phases of bent-shaped bimesogens, is discussed.

cond-mat.soft↗

arXiv2020

The role of intermolecular interactions in stabilizing the structure of the nematic twist-bend phase

The relationship between the molecular structure and the formation of the NTB phase is still at an early stage of development. This is mainly related to molecular geometry, while the correlation between the NTB phase and the electronic structure is ambiguous. To explore the electronic effect on properties and stabilization of the NTB phase we investigated 2,3-difluoro-4,4-dipentyl-p-terphenyl dimers (DTC5Cn). We used IR polarized spectroscopy, which can at least in principle, bring information about the ordering in NTB phase. All dimers show a significant drop of the average value of the transition dipole moment dμ/dQ for parallel dipoles at the transition to the NTB phase, and an increase for perpendicular dipoles, despite its remaining unchanged for the monomer. These results coincide well with DFT simulations of vibrational dipole derivatives for molecules assembled in pseudo-layers of the NTB phase. The DFT calculations were used to determine the geometric and electronic properties of the hydrogen bonded complexes. We have provided experimental and theoretical evidence of stabilization of the NTB phase by arrays of multiple hydrogen bonds (XF...HX, X-benzene ring).

physics.chem-ph↗

arXiv2020

The interplay between spatial and heliconical bond order in twist-bend nematic materials

The nanostructure of two novel sulfur containing dimer materials has been investigated experimentally by hard and by resonant tender X-ray scattering techniques. On cooling the dimers through the nematic to twist-bend nematic (N-NTB) phase transition, the correlation length associated with short-range positional order drops, while the heliconical orientational order becomes more correlated. The heliconical pitch shows a stronger temperature dependence near the N-NTB transition than observed in previously studied dimers, such as the CBnCB series of compounds. We explain both this strong variation and the dependence of the heliconical pitch on the length of the spacer connecting the monomer units by taking into account a temperature dependent molecular bend and intermolecular overlap. and. The heliconical structure is observed even in the upper 3-4°C range of the smectic phase that forms just below the NTB state. The coexistence of smectic layering and the heliconical order indicates a SmCTB -type phase where the rigid units of the dimers are tilted with respect to the layer normal in order to accommodate the bent conformation of the dimers, but the tilt direction rotates along the heliconical axis. This is potentially similar to the SmCTB phase reported by Abberley et al (Nat. Commun. 2018, 9, 228) below a SmA phase.

cond-mat.soft↗

arXiv2020

Molecular biaxiality determines the helical structure -- Infrared measurements of the molecular order in the NTB phase of difluoro terphenyl dimers

IR polarized spectroscopy was employed, to obtain the three components of IR absorbance for a series DTC5Cn (n=5,7,9,11) of bent-shaped dimers containing double fluorinated terphenyl core. Then, they were used to calculate both uniaxial (S) and biaxial (D) order parameters, for various molecular groups of dimers. The molecule bend was estimated due to observed difference between S parameters for the terphenyl core and central hydrocarbon linker. It is found to be significant in N phase despite almost no macroscopic director bend. The temperature dependence of the order parameter, S, distinctly reverses its monotonic trend of increase to decrease at the transition temperature, TN-NTB to the NTB phase as result of the director tilt in the NTB phase. The biaxial order parameter, D, is negligible in the N phase, and then starts increasing on entering the NTB temperature, similarly to sin2q. The local director curvature is found to be controlled by the molecular biaxiality parameter.

cond-mat.soft↗

arXiv2020

Feasibility Analysis and Preliminary Design of ChipSat Entry for In-situ Investigation of the Atmosphere of Venus

Recent miniaturization of electronics in very small, low-cost and low-power configurations suitable for use in spacecraft have inspired innovative small-scale satellite concepts, such as ChipSats, centimeter-scale satellites with a mass of a few grams. These extremely small spacecraft have the potential to usher in a new age of space science accessibility. Due to their low ballistic coefficient, ChipSats can potentially be used in a swarm constellation for extended surveys of planetary atmospheres, providing large amounts of data with high reliability and redundancy. We present a preliminary feasibility analysis of a ChipSat planetary atmospheric entry mission with the purpose of searching for traces of microscopic lifeforms in the atmosphere of Venus. Indeed, the lower cloud layer of the Venusian atmosphere could be a good target for searching for microbial lifeforms, due to the favourable atmospheric conditions and the presence of micron-sized sulfuric acid aerosols. A numerical model simulating the planetary entry of a spacecraft of specified geometry, applicable to any atmosphere for which sufficient atmospheric data are available, is implemented and verified. The results are used to create a high-level design of a ChipSat mission cruising in the Venusian atmosphere at altitudes favorable for the existence of life. The paper discusses the ChipSat mission concept and considerations about the spacecraft preliminary design at system level, including the selection of a potential payload.

astro-ph.IM↗

arXiv2019

Probing Molecular Ordering in the Nematic Phases of para-Linked Bimesogen Dimers through NMR Studies of Flexible Prochiral Solutes

The quadrupolar splittings of perdeuteriated n-decane dissolved in nematic phases formed by mesogenic dimers of the CBnCB series, for n=7,9,10,11 are measured throughout the entire temperature range of these phases. The results of the measurements, are reported together with related measurements using the common nematic phase of 5CB as a solvent for n-decane. The data obtained from the $^{13}$C spectra of the cyanobiphenyl mesogenic units of the monomeric and dimeric solvent molecules yield the order parameter of those units. The information obtained from this set of experiments is used to elucidate the structure of the low temperature (N$_X$) and the high temperature (N) nematic phases of CBnCB dimers with n=7,9,11. The polar twisted nematic (N$_{PT}$) model is found to provide a consistent description not only of these measurements, but also of NMR measurements previously reported in the literature for these phases. These findings suggest that the high temperature nematic (N) is not a common, locally uniaxial and apolar nematic, but rather a nematic phase consisting of N$_{PT}$ clusters. The twist-bend (N$_{TB}$) model, often identified with the N$_X$ phase, is shown to be inadequate to account even qualitatively for crucial features of the experimental findings.

cond-mat.mtrl-sci↗

arXiv2019

Asteroid mining with small spacecraft and its economic feasibility

Asteroid mining offers the possibility to revolutionize supply of resources vital for human civilization. Preliminary analysis suggests that Near-Earth Asteroids (NEA) contain enough volatile and high value minerals to make the mining process economically feasible. Considering possible applications, specifically the mining of water in space has become a major focus for near-term options. Most proposed projects for asteroid mining involve spacecraft based on traditional designs resulting in large, monolithic and expensive systems. An alternative approach is presented in this paper, basing the asteroid mining process on multiple small spacecraft. To the best knowledge of the authors, only limited analysis of the asteroid mining capability of small spacecraft has been conducted. This paper explores the possibility to perform asteroid mining operations with spacecraft that have a mass under 500 kg and deliver 100 kg of water per trip. The mining process considers water extraction through microwave heating with an efficiency of 2 Wh/g.The proposed, small spacecraft can reach NEAs within a range of $\sim 0.03$ AU relative to earth's orbit, offering a delta V of 437 m/s per one-way trip. A high-level systems engineering and economic analysis provides a closed spacecraft design as a baseline and puts the cost of the proposed spacecraft at $ 113.6 million/unit. The results indicate that more than one hundred spacecraft and their successful operation for over five years are required to achieve a financial break-even point. Pros and cons of using small spacecraft swarms are highlighted and the uncertainties associated with cost and profit of space related business ventures are analyzed.

astro-ph.IM↗

arXiv2014

Do the short helices exist in the nematic TB phase?

Dimeric compounds forming twist-bend nematic, Ntb, phase show unusual optical texture related to the formation of arrays of focal conic defects. Some of the focal conics show submicron internal structure with 8 nm periodicity, which is very close to that found in the crystalline phase of the material, suggesting surface freezing.

cond-mat.soft↗